For an image pickup apparatus such as a video camera and digital still camera, an auto focusing function is one of the important functions for improving operability. One of typical methods of realizing the function is called “hill-climbing method.” This method extracts a medium/high frequency component from an image signal obtained by image taking and controls the lens position so that the level becomes a maximum. This system is based on the principle that a circle of confusion increases as the lens goes away from a focal position and the contrast of the object image formed through the lens decreases as it goes away from the focal position. The medium/high frequency component of the image signal is a signal that corresponds to the degree of the contrast of this object image.
This system is classified as a passive system, which requires no dedicated light-emitting apparatus as compared with an active system such as an “infrared system,” . This system further has a feature of being capable of high precision focusing because it receives little influence from the distance from the object. Furthermore, since the hill-climbing method uses an image signal itself, it requires no other optical system which is required by another passive system called a “phase difference detection system.” For this reason, this system provides cost reduction and miniaturization.
On the other hand, this system leaves much to be desired in handling objects and scenes such as (1) objects with low contrast whose focus signal level relatively decreases, (2) scenes with a mixture of far and near objects with a plurality of maximum points of a focus signal and (3) scenes under low illumination susceptible to noise in an image signal, etc.
A conventional configuration of a hill-climbing type auto focusing apparatus will be explained with reference to drawings.
FIG. 7 shows a configuration of a conventional auto focusing apparatus.
An image-taking lens 1 made up of a plurality of lenses including a focusing lens 1a is position-controlled by a lens driving section 6 (e.g., stepping motor and its driving circuit). An optical image of an object is formed on an image pickup element 2 (e.g., CCD) which becomes image pickup means through the image-taking lens 1.
An image pickup element 2 photoelectrically converts the formed object image and outputs it as a time-series signal. An image signal generation circuit 3 applies various types of signal processing to the output of the image pickup element 2 and outputs a predetermined image signal CO (e.g., NTSC signal). Here, the various types of signal processing refer to analog/digital conversion, gain control, γ correction, brightness signal generation processing, color-difference signal generation processing, etc., and further include aperture correction, noise reduction, etc., as required.
A focus signal detection circuit 4 integrates a brightness signal YE out of the time-series signal output from the image signal generation circuit 3 using a low pass filter 41 (hereinafter referred to as “LPF”), removes the noise component and outputs a BP signal which has been differentiated by a high pass filter 42 (hereinafter referred to as “HPF”).
A peak detection circuit 43 converts this signal to an absolute value, detects a peak value (PK signal) of a signal corresponding to a preset area (e.g., central 50% area of image-taking screen) in every horizontal scanning period and an addition circuit 44 further adds up these peak values for a vertical scanning period to generate a focus signal VF. This focus signal VF becomes a representative field value corresponding to the degree of the contract of the object image.
Here, FIG. 8 shows a schematic view illustrating an image of the operation of the focus signal detection circuit 4 for detecting a focus signal VF from the image pickup screen. The same figure shows an example of an object having vertical stripes of “white, black, white.” FIG. 8(a) shows an out-of-focus state of the object and FIG. 8(b) shows an in-focus state of the object.
In FIG. 8(a), when the object is out of focus as shown in the first illustration from the left, the signal level of the detection area 32 which is a substantially central 50% area of the image-taking screen 31 for a horizontal scanning period is detected and differentiated by the HPF 42 and the resulting BP signal is as shown in the second illustration from the left. When this signal is converted to an absolute value by the peak detection circuit 43, the resulting signal is as shown in the third illustration from the left and the peak value (PK signal) at that time is output to the addition circuit 44. The fourth illustration from the left indicates the peak value by a fine line arrow and the length thereof indicates the magnitude of the peak value. Likewise, peak values in the detection area 32 are detected for every horizontal scanning period, and the addition circuit 44 adds up those peak values for a vertical scanning period to obtain a focus signal VF. The magnitude of the focus signal VF is indicated by a bold line arrow in the fourth illustration from the left. The length of this bold line arrow indicates the magnitude of the focus signal VF.
Then, in an in-focus state when the object is in focus as with the first illustration from the left in FIG. 8 (b) a BP signal obtained by detecting and differentiating the signal level of the detection area 32 which is the substantially central 50% area on the image-taking screen 31 for a horizontal scanning period by the HPF 42 is as shown in the second illustration from the left. When the peak detection circuit 43 converts this signal to an absolute value, the resulting signal is as shown in the third illustration from the left and the peak value (PK signal) at that moment is output to the addition circuit 44. The fourth illustration from the left indicates the peak value using a fine line arrow and the length thereof indicates the magnitude of the peak value. Likewise, peak values in the detection area 32 are detected for every horizontal scanning period, and the addition circuit 44 adds up those peak values for a vertical scanning period to obtain a focus signal VF. The magnitude of the focus signal VF is indicated by a bold line arrow in the fourth illustration from the left. The length of this bold line arrow indicates the magnitude of the focus signal VF.
Thus, there is a difference in the signal level detected by the HPF 42 between the out-of-focus state and in-focus state and a focus signal resulting from the addition of peak values of this signal naturally has a difference. As shown in this figure, the focus signal VF in the in-focus state is greater than that in the out-of-focus state.
Returning to FIG. 7, the lens control circuit 5 generates a variation component ΔVF by calculating a difference between this focus signal VF and a past focus signal, for example, a focus signal obtained one field ahead by a differential circuit 501. Seeing the sign of this variation component ΔVF, an in-focus direction decision circuit 502 decides whether the in-focus direction is on a far side or near side relative to the actual point or whether it is the same as or opposite to the immediately preceding moving direction. A lens control amount calculation circuit 503 adds a predetermined amount of movement to this moving direction and outputs the result as an amount of lens control to the lens driving section 6. The lens driving section 6 drives the focusing lens 1a based on this amount of control. Focusing is automatically performed by these configurations and operations.
Here, the amount of lens movement at the lens control amount calculation circuit 503 in the lens control circuit 5 will be explained in more detail.
When the amount of movement of the focusing lens 1a is increased, the moving speed of the focusing lens 1a increases. However, when the moving speed is too high, the stepping motor cannot keep track of the correlation between the amount of lens control and the moving position, possibly causing a so-called out-of-synchronization phenomenon. In addition, when the amount of movement of the focusing lens 1a is too large, a hunting phenomenon in which the lens moves back and forth a great deal around the in-focus position becomes noticeable, considerably deteriorating the quality of the image. On the contrary, when the amount of movement is too small, it takes quite a long time to reach the in-focus position, deteriorating responsivity.
Therefore, a method of deciding the amount of lens movement according to the level of the focus signal VF is considered. FIG. 9 shows a hill-climbing curve of a focus signal VF which changes depending on the object. In FIG. 9, the X-axis shows the lens position of the focusing lens 1a and shows the focal position substantially at the center. The Y-axis shows the level of the focus signal VF. The characteristic of an object A shows a characteristic when the image-taking condition is good (when contrast and illumination are sufficient) , while the characteristic of an object B shows a state in which the image-taking condition of the object is bad (low contrast, low illumination, etc.). LEV1 and LEV2 show threshold levels and MV1 to MV3 show amounts of movement of the lens. For example, as shown in FIG. 9, the threshold level LEV1 and threshold level LEV2 (LEV2>LEV1) are specified beforehand, and if the relationship between the threshold level LEV1, threshold level LEV2 and focus signal VF is VF<LEV1, then the amount of movement is assumed to be MV1 and if LEV1≦VF<LEV2, the amount of movement is assumed to be MV2 and if LEV2≦VF, the amount of movement is assumed to be MV3. Here, suppose the relationship between the amounts of movements MV1 to MV3 is MV1>MV2>MV3. FIG. 9 indicates the amount of movement of MV1 to MV3 by the length of the arrow.
In FIG. 9, in the case of the object A, for example, when the lens position is P1, the focus signal VF is less than LEV1, and therefore the lens moves to the in-focus position by the amount of movement MV1. When the lens moves to the in-focus position and the level of the focus signal VF increases and exceeds LEV1 (the lens position at this time is P2), the lens moves to the in-focus position by replacing the amount of movement MV1 by MV2 which is a smaller amount of movement than MV1. Furthermore, when the level of the focus signal VF exceeds LEV2 (the lens position at this time is P3) , the lens moves by replacing MV2 by MV3 which is a smaller amount of movement than MV2. Then, when the lens is moved to P4 past the in-focus position, the level of the focus signal VF is decreased, and therefore the lens is moved backward and the lens is brought closer to the in-focus position. The position where the level of the focus signal VF reaches the highest point is the in-focus position. Thus, changing the amount of movement of the focusing lens 1a according to the level of the focus signal VF solves the above described problem.
However, the level of the focus signal VF varies depending on the condition (contrast and illumination, etc.) of the object even when the distance from the object is the same. Therefore, as with the object B in FIG. 9, for example, even if the focusing lens 1a is near the in-focus position, the amount of movement becomes MV2 which is greater than MV3 and the quality of the image (moving image) may be damaged by hunting.
Furthermore, when a high resolution still image is taken by a still image taking function of a digital still camera or video camera, it is preferable from the standpoint of resolution, etc., that the object image taken when the shutter is released be an object image exposed to light while the lens is stopped. Furthermore, as the number of pixels of the still image increases, higher resolution is required, and therefore higher in-focus accuracy is required.
FIG. 10 is a schematic view of the behavior of the lens movement in the case of the object B shown in FIG. 9 and the horizontal axis shows the time until the lens is stopped and the vertical axis shows the lens position. In the conventional configuration, the accuracy of stopping of the lens strongly depends on the predetermined amount of lens movement (MV1 to MV3) and it is not possible to reduce the amount of movement for the same reason as that of a moving image, and therefore the accuracy of stopping may deteriorate and the quality (especially resolution) of the still image may deteriorate.
In the conventional configuration, the lens is stopped after a predetermined time.